In many applications knowledge of the position of an audio source is beneficial and may e.g. be used to optimize the signal processing of audio captured from the audio source. For example, the signal processing for hands-free communication and teleconferencing applications may be optimized dependent on the specific position, and typically just the angular direction, of the sound source. Accordingly, many audio processing systems comprise audio tracking systems that allow the (e.g. angular) position of a sound source to be detected and tracked.
One approach for determining a position of an audio source uses a microphone array with the relative differences between the microphone signals being analyzed to estimate the position of the source. Common localization methods using microphone arrays are mainly based on calculating the time-difference of arrival of sound waves on each of the microphones.
Other localization methods using closely spaced microphones are based on constructing first order differential responses by combining two microphone signals and using an optimization criterion to steer a null in the direction of the sound source.
Using three microphones, the location of a sound source with respect to the array can be determined in the 360-degree (horizontal) azimuthal plane based on the time of arrivals, and indeed based on the differences in the time of arrival. However, in order to reduce the cost and complexity of the associated processing, it is desirable to reduce the number of microphones as much as possible, and it is therefore desirable to perform position determination using only two microphones.
With two microphones, a proper time of arrival calculation may allow determination of specific positions, i.e. position determination in typically the horizontal plane. However, such calculations require the time of transmission from the audio source to be known and thus typically require the audio source to be synchronized with the position determining circuitry. This is typically highly impractical and therefore position determination is typically based on the difference in time of arrival measurements between the microphones. For a two microphone implementation this means that only the angular direction can typically be determined with the distance to the sound source not being known. However, for many applications such an angular position determination is highly advantageous and indeed is sufficient for many applications.
However, another problem with a two microphone setup is that it is completely symmetric around the axis interconnecting the two microphones as illustrated in FIG. 1. In the example, two microphones M1 and M2 are used to determine the angular direction A to a sound source S based on a time of difference between the two microphones M1 and M2.
Thus, the system determines the time difference of arrival between the wave-fronts for the microphones M1 and M2. If the source is located in the far-field, then the sound waves can be assumed to be planar and parallel to each other. Using trigonometry, the angle is related to the Time Difference Of Arrival (TDOA) t (in seconds) by
                    t        =                              d            c                    ⁢                      cos            ⁡                          (              A              )                                                          (        1        )            where d is the inter-microphone spacing, and c is the speed of sound in air. The angle A can therefore be determined by
                              A          =                                    cos                              -                1                                      ⁡                          (                              tc                d                            )                                      ,                            (        2        )            where A is in the range [0,180°].
However, this approach has an inherent ambiguity and can only determine A in the range [0,180°]. Thus, it does not provide sufficient information about the direction of the source in the 360-degree azimuthal plane. Specifically, it cannot differentiate between whether the sound source is located at position S or at the phantom position G.
Hence, an improved approach for sound source position estimation would be advantageous and in particular an approach allowing increased flexibility, facilitated operation and/or implementation, lower complexity, reduced cost, reduced computational requirements and/or improved performance would be advantageous. In particular, an approach suitable for improved sound source position determination, and especially allowing ambiguity resolution, for a two microphone setup would be advantageous.